Modules for gesture recognition become more and more important in various application areas.
There are different solution approaches for this technology, one of which is the scanning of a field with a line or a strip of a radiation. Typically, the line or the strip is generated by infrared light. The radiation is scattered at the objects in the scanned space and captured by a camera system. Then, this information is evaluated, partially as 2-D-information and partially as 3-D-information.
Several types come into consideration as a light source. Especially, laser diodes are of great interest for these applications. Laser diodes can be constructed to be very small and are efficient radiation sources. Particularly, compact modules are of special interest because the modules are to be installed, for example, in laptops, smartphones or other devices of information technology.
A solution in the state of the art consists in a laser diode generating a radiation (in the IR range) incident on an optical element having two effective surfaces, wherein the first (rotationally symmetric, and often aspheric) surface collimates the radiation and the second (often also aspheric) surface is formed as a “cylinder” with a positive refractive power and generates a line or a strip. A micromechanical mirror is arranged behind the focal point or the focal line of the cylindrical surface, which realizes the scanning of the line which will then propagate divergently into the space.
The above-mentioned structure may, in fact, realize a very compact module, but it is not possible to obtain uniform line intensity. A uniform line intensity may be obtained for a given radiation characteristic of the source by an appropriate design of the cylinder geometry, but the practice shows that the laser diodes have relatively strong fluctuations in their radiation pattern which then will result in strong fluctuations in the intensity over the line, particularly, if a large portion of the diode radiation shall be used for the line generation.
A further problem in using a single cylinder surface is that it generates a focal line in the space whose width is a few hundredths millimeters in the focus (viewed for a pupil in a distance of 100 mm with an extension of 7 mm). Thus, the current demands on the eye security of the module reduce the performance of the light source which, in turn, is critical for the detection of the signal.
By default, the problem of uniform line intensity is solved by using not a single cylinder surface but a stack or an array of cylinder surfaces, each single cylinder surface distributing the radiation on the line. This approach permits to achieve a good homogenization for different radiation patterns. Nevertheless, the beam cross section behind the cylinder lens stack is increased. Therefore, disadvantageously, a larger surface is required for the deflection mirror for scanning the radiation in the space, which can hardly be realized in practice.
Conventional devices for generating a line or strip pattern for gesture recognition using a pivotable micromechanical mirror (also referred to as “Micro electro mechanical systems” (Mems)) are schematically illustrated in FIGS. 1 and 2.
The devices for generating a line pattern according to FIGS. 1 and 2 comprise a diode laser 30 and a lens 20. The first surface of the lens 20 collimates the light exiting divergently from the laser 30. The collimated light passes through the second surface of the lens 20 which, due to its cylindrical form, generates a line or a spatially extended strip in the image plane 56 from the collimated radiation. When using a single cylinder surface of FIG. 1, a line shaped focus is generated in the region of the optical axis. The line-shaped radiation propagates divergently in the space behind the focus and generates a finitely propagated line or strip pattern in the region of a plane 50 at which, for example, an object for gesture recognition may be located. From the backscattering of an object located in the region of the plane 50, if necessary, one can infer the structure of the object and the face structures (gestures) that may be present in the object. To detect objects located outside the plane 50, a micromechanical mirror 40 is arranged behind the line-shaped focus, which accomplishes the scanning of the line pattern in the space. In the present case, the rotation axis of the micromechanical mirror 40 is vertical to the optical axis in the plane of the paper. Thus, in fact, the line pattern is not—as schematically illustrated—projected onto the plane 50 but rather into the space by the mirror 40. In this respect, the plane 50 serves only for illustrating the beam course formed behind the lens 20 without the mirror 40. The nominal tilt angle of the mirror with respect to the optical axis is usually in the range of 40 to 50 degrees, and the mirror tilts about this average in the range of +/−10 to +/−25 degrees.
As mentioned above, the use of a single cylinder surface of FIG. 1 is disadvantageous insofar as hereby, on the one hand, the fluctuations of the intensity of the laser diode 30—as viewed with respect to the radiation angle of the laser diode—will affect substantially the line pattern and, on the other hand, the demands on the eye security will result in an unwanted limitation of the intensity of the laser diode 30. This, in turn, is critical for the detection of the backscattered signal (higher demands on the detector).
The use of a cylinder lens stack of FIG. 2 has the important disadvantage that it causes an important increase of the beam cross section behind the cylinder surface stack because not a single line-shaped focus is generated in the region of the optical axis (FIG. 1), but a plurality of line-shaped focuses (FIG. 2) whose respectively divergent partial radiations will then overlap to form a strip. Disadvantageously, this requires a larger surface for the deflection mirror 40 for scanning the radiation in the space, which can hardly be realized in devices such as smartphones and places great demands on the deflection mirror 40.
Further U.S. Pat. No. 5,808,775 A, US 2004/0247011 A1, U.S. Pat. No. 6,215,598 B1 and US 2014/0328075 A1 disclose lenses having an optical surface comprising a plurality of cylinder-shaped partial areas.